Amorphous (noncrystalline) oxide semiconductors for use in TFTs have higher carrier mobility and larger optical band gaps and can be deposited at lower temperatures than those of general amorphous silicon (a-Si) semiconductors. For these reasons, the amorphous oxide semiconductors are expected to be applied typically to next-generation displays which should have large sizes and high resolutions and should be driven at high speed; and to resin substrates which have poor thermal stability. Deposition of such an oxide semiconductor (film) preferably employs sputtering, in which a sputtering target including the same materials as the film is subjected to sputtering. The resulting thin film deposited by sputtering advantageously excels in in-plane uniformity typically of chemical composition and film thickness in a film-plane direction (in-plane) and can advantageously have the same chemical composition as the sputtering target, as compared to thin films deposited by ion plating, vacuum vapor deposition, or electron beam physical vapor deposition. Sputtering targets are generally formed by mixing oxide powders, sintering the resulting mixture to give a sintered compact, and machining the sintered compact.
Exemplary oxide semiconductors for use in display devices have compositions corresponding to indium-containing amorphous oxide semiconductors [e.g., In—Ga—Zn—O, In—Zn—O, and In—Sn—O (indium-tin oxide; ITO)]. These oxide semiconductors, however, employ indium, which is a rare metal, and may cause higher material cost in mass production process. To avoid this disadvantage, there have been proposed zinc-tin oxide (ZTO) semiconductors which are amorphized by adding Sn to Zn. These have been proposed as oxide semiconductors that do not contain expensive indium, have lower material cost, and are suitable for mass production. PTLs 1 to 4 disclose sputtering targets which are useful for the production of the ZTO semiconductor films.
Among them, PTL 1 proposes a technique of performing sintering for a long time to control the structure to contain no tin oxide phase, so as to suppress the generation of abnormal discharge and cracking during sputtering. PTL 2 proposes a technique of performing two production steps of preparing a calcined powder (partially sintered powder) at a low temperature of 900° C. to 1300° C.; and firing the calcined powder to give a high-density ZTO sintered compact as a sputtering target, so as to suppress abnormal discharge during sputtering. PTL 3 proposes a technique of incorporating a spinel-type AB2O4 compound to give a sputtering target having better electroconductivity and a high density. PTL 4 proposes a technique of performing two production steps of preparing a calcined powder at a low temperature of 900° C. to 1100° C., and firing the calcined powder to give a dense ZTO sintered compact.
PTL 5 proposes a ZTO sputtering target containing indium in a low content, as a sputtering target which is used for the deposition of transparent conductive films and which has a low resistivity and a high relative density even though having an indium content lower than that of an ITO. In general, reduction in indium content in an ITO causes the resulting sputtering target to have a lower relative density and a higher bulk resistivity. However, the technique disclosed in PTL 5 gives a sputtering target which has a high density and a low resistivity and can less suffer from abnormal discharge upon sputtering, owing to the coexistence of a bixbyite structure compound expressed by In2O3 with a spinel structure compound expressed by Zn2SnO4.